The application relates generally to printable, magnetizable inks, to substrates printed with such inks, and to formulating, printing, and temporarily or permanently magnetizing such inks to magnetically support loads with a significant magnetic load to ink thickness ratio. The application further relates to such inks that can be or are overprinted with high resolution images and/or indicia.
Material having magnetic properties may be incorporated into a variety of applications. For instance, manufacturers have incorporated magnetic material into educational, instructional and interactive devices for children. Magnets and devices having magnetic properties have a special appeal due to the invisible properties of magnetism. There are numerous types of interactive toys, games, appliances, and displays, in which material having magnetic properties is used to advantage to freely move magnetically attached objects, or toys. Also, there are many applications whereby magnetism is used to magnetically connect objects with objects, and surfaces to surfaces.
One method of incorporating the invisible properties of magnetism into a product involves adding a ferromagnetic material, such as iron particles, into conventional paints. The iron particles are blended, or mixed, into the paint to form a temporarily magnetizable paint. The temporarily magnetizable paint is then applied in the same way as ordinary paint to the surface of a substrate, such as wall board, wood, sheet rock, plywood, or the like to make signs and other types of displays having a surface to which a magnet can attract itself. A disadvantage with this approach is that paint is commonly applied by hand and the thickness of the paint applied is then virtually impossible to control.
In addition, the formulations used in practice are low in Fe density due to the fact that the viscosity becomes too high with concentrations of Fe powder over about 60 or 70%, and are messy to apply. Further, particle distribution is often poor, necessitating repeated coatings to ensure useful magnetic interaction. A further limitation is that conventional temporarily magnetizable paints were not paired with specific permanently magnetized materials or elements to be used in conjunction with the temporarily magnetizable paint. The permanently magnetized element might be indicated, but only in very broad terms, such as “use with rubber magnets of about 0.5 mm thick,” or “use with rare Earth magnets.” The permanently magnetic objects, permanently magnetized rubber magnets, or the like need to be specified with very wide tolerances to accommodate the variations in the method of application of the paint. As a result, the magnetic efficiencies were far lower than should theoretically be obtainable with available magnetic materials. A further disadvantage of using the magnetic paint described above is that a second process is required to apply images to the magnetic paint. The means to do this can necessitate using colored paints and then painting images onto the magnetic paint—or by covering with wallpaper.
Another previously proposed way of incorporating the invisible properties of magnetism into a product involves positioning metal plates between substrates. See, for example, U.S. Pat. No. 5,852,890 (Pynenburg). This involves a highly labor intensive production process and is not efficient in terms of use of the temporarily magnetizable material. The limit is typically the minimum metal sheet thickness that is commercially available. Applications using metal sheets are also limited by cost, and safety concerns due to sharp edges of the thin metal sheets. Also, efficiency is limited due to the fact that the permanently magnetizable materials that are to be used according to Pyenburg are not specified to any degree.
U.S. Pat. No. 4,702,700 (Taylor) proposes a book with sheets of magnetic material embedded within the pages, which attract removable magnetic pieces placed onto the surface of the page. Taylor's magnetic sheets are thick and produce a significant bulge in the pages. The bulge is esthetically unattractive, and spoils the invisible effect of the magnetism by making it obvious that there is a concealed artifice within the pages. The problem can be overcome by adding compensating fillers. Taylor's invention requires hand-assembly which is a major limitation. It is believed that the weight of the magnetic sheets used by Taylor would also be such as to restrict the number and size of the sheets that could practically be included in one book.
U.S. Pat. No. 6,159,577 (Pynenburg et al.) describes a modifiable sign system where off-the-shelf temporarily magnetizable inks are silkscreen coated at a preferred thickness of 0.025 mm over the whole surface of a plastic substrate of at least 0.25 mm thick, then 100% overcoated with ultraviolet-curable white of thickness less than 0.1 mm, then silkscreen overprinted with colored ink of thickness less than 0.1 mm. There is a great lack of efficiency in this proposal in that very thick extruded rubber magnets of thicknesses of 0.6 mm to 1.5 mm must be used with the relatively inefficient, off-the-shelf, temporarily magnetizable inks. Substrates are plastic at thicknesses of at least 0.25 mm which limits the methods by which the temporarily magnetizable surfaces can be overprinted. For example, substrates of this thickness could not be printed on a sheet offset lithographic press, or by gravure, or flexo for example. Further, in this system both the temporarily and the permanently magnetizable layers are relatively stiff. As a result, any departure from flatness may result in gaps between the two magnetic components that cannot be taken up by flexing of either component. Such gaps result in significant reduction in the magnetic load that can be supported.
U.S. Pat. No. 3,998,160 (Pearce) describes a method of printing an ink containing magnetic particles and aligning the particles magnetically prior to printing such that remanence patterns can be read by sensing heads to identify forgeries in bank notes and other security items. Pearce does not describe or suggest using magnetizable inks to magnetically support loads. Further, Pearce does not suggest combining two surfaces with magnetic printing inks or coatings such that permanently magnetized inks or coatings magnetically interact with either permanently or temporarily magnetized inks or coatings.
U.S. Pat. No. 5,525,649 (Nishimura et al) describes formulating a magnetic paint with regularly dispersed fine particles to reduce noise levels in recording analog or digital data magnetically.
U.S. Pat. No. 5,869,148 (Silverschotz et al.) describes a process for the in-line, high speed manufacturing of magnetic products where a slurry of a permanently magnetizable material suspended in a binder is applied to a substrate at thicknesses of from 0.1 mm to 0.5 mm, dried, and then permanently magnetized with a coil inductor at 10,000 Oerstedt. The pole line spacing is 1.5 mm to 2.5 mm. An SrFe concentration of 64% is indicated. For reverse-roller coating, a viscosity 300 to 5,000 cps, preferably 3,000 to 4,000 cps, is described. The coating is a continuous process on a moving web. Silverschotz's slurry is not printable, it can only be coated onto the entire surface of a substrate. Printing is understood as allowing the positioning of ink in selected areas on a substrate to form meaningful shapes and images.
U.S. Pat. No. 6,853,280 (Sugawara) describes a method of magnetizing magnetic sheets using rare Earth permanent magnet roller configuration in which sectors of a roller may be magnetized radially, so that poles are adjacent and not opposing on the outside of the roller. The magnetizing roller produces a field of 6,000 Gauss.
U.S. Pat. No. 5,942,961 (Srail et al.) describes an apparatus for permanently magnetizing magnetic sheets with rollers formed from stacks of disks. Each disk is Magnetized axially, with the poles of adjacent disks opposed to produce an effective external pole between them. Srail uses upper and lower rollers on opposite sides of the material being magnetized, with complementary magnetic pole patterns.
U.S. Pat. No. 5,843,329 (Deetz) describes a magnetic paint additive in broad terms, where a wide range of iron particle sizes are suspended within surfactants that can be added to paints. A surfactant is a wetting agent that lowers the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids. At one point, the composite magnetic paint additive is stated to contain in the order of 8,000 grams of iron powder per gallon, or about 80% iron powder by weight. Deetz claims that the additives, including the surfactant, do not increase a paint's viscosity by more than 25%. Deetz describes many ways of formulating the magnetic paint additives but does not describe the magnetic load supporting characteristics, or optimum magnetic fields, of the dried magnetic paints or coatings. Example 4 describes coated thicknesses of between 1 to 6 mils (0.02 mm to about 0.25 mm). Deetz describes the possible use of silk screening and sprays to apply magnetic paints. It appears that the silk screen is intended only to assist in controlling the thickness and uniformity of a coating, not as an image-forming printing process. Deetz describes particle size selection on the bases of the surface characteristics required of paints or coatings. Deetz describes coating between substrates. Deetz describes larger particles as yielding stronger magnetism and recommends a broad range of particle sizes. Deetz states that any type of Fe particle can be used. One Example describes a coated substrate less than 10 mils (0.25 mm) thick laminated with a second surface sheet.
U.S. Pat. No. 3,503,882 (Fitch) discloses a paint composition containing iron powder and an epoxy ester resin with an emulsifiable polyethylene wax and an organophilic alkyl ammonium bentonite dispersed in a paint hydrocarbon solvent when applied to a substrate and dried, a surface to which magnetic symbols will adhere and which will accept chalk markings. The iron powder employed was 100 to 200 mesh (0.005 to 0.01 in., or 0.125 to 0.25 mm) with over half above 200 mesh and comprising by weight about 70% to 85% iron powder to epoxy ester resin. The product was brushed on.
U.S. Pat. No. 5,587,102 (Stern et al.) discloses a magnetic latex paint composition comprising a carrier, particulate magnetically permeable material, a binder and a thickening agent having thixotropic and viscosity characteristics such that the paint composition has high viscosity when stationary, and low viscosity when subject to shear forces while being painted on a wall. Particulate iron no smaller than 350 mesh (70 μm) was employed with synthetic clay as a thickening agent to keep particles in suspension. Thus formulated, drying retarders were necessary so that the smooth surface after paint application could be achieved without lap marks.
U.S. Pat. No. 5,949,050 (Fosbenner) proposes magnetic cards containing, sandwiched within them, a shaped sheet of magnetic material that produces an image by attracting magnetic particles in a liquid imaging cell. The shaped sheets of magnetic material are set into correspondingly shaped cutouts in a filler sheet in the cards. Fosbenner suggests that “a magnetic or magnetizable ink” could be used instead of magnetic sheets, but provides little or no disclosure of how to formulate or apply such a magnetic ink. Because of the use of filler sheets, Fosbenner's cards are thick. The filler sheets also add to the bulk and weight.
My own earlier U.S. Pat. No. 7,192,628 (Burrows '628) describes spot printing magnetizable inks that are thin enough to be compressed into a thin card substrate, so that they can be directly offset overprinted, and yet still magnetically support useful loads. The highest efficiency measured based on Burrows, for one permanently magnetized layer and one temporarily magnetized layer, is in the order of a combined thickness of 0.7 mm to support loads of almost 0.4 grams per square centimeter using multi-polar fields.
Burrows '628 describes in an Example a permanently magnetizable ink based on 79% of commercially available strontium ferrite with a nominal particle size of 2 μm±0.5 μm, in a styrene-butadiene carrier thinned with mineral spirits. In practice, that ink when formulated with available ingredients was found to have a very high viscosity, over 50,000 cps. The high viscosity renders that ink slow and difficult to print with, and tending to clog the printing machinery. Although that ink has been used commercially, it left room for further improvement.
The magnetically soft iron inks in Burrows '628 use much larger particles, around 50 microns, to improve the magnetic properties. The large particles compel a comparatively thick ink layer, to contain the large particles, and cause the ink layer to have a surface too rough for direct high-quality printing. Further, the “double scrubbing” process specified for the Fe particles in Burrows '628 is a process of wet pressing and grinding. Pressing creates agglomerated particles with high surface porosity, and thus high surface area. Grinding creates jagged particles with high surface area.